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Appendix II.


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i I 1


P. 256. Turning Balls.—Brass and gun-metal balls are
much used for feed-pump- and safety-valves on account of
absence of sticking which they ensure. To turn them in the
lathe, a cup.chuck in hard wood—A, Fig. 790—is provided, into-

which the ball is fixed. This form of chuck permits the work
to be constantly changed in position; and, if this be carefully
done, it will be clear that any section will be a circle, which is